The Forgotten Realms has long been a favorite supported setting of mine. When 4E came along and mucked things up about I wasn't too thrilled. I liked some of the concepts, but my biggest gripe was the displacement of land and some of the changes that happened behind the scenes. I haven't read The Sundering novels and I really should before I completely devote myself to this project.
The idea with this project is to build forward from The Tearing of The Weave. One thing I never cared for was that there have been NPCs that characters might try to pester for help. And if the characters are established enough, perhaps the NPCs may consider an audience with them. One of the immediate goals is the to eliminate NPCs above 20th level. Elminster, while I love him as an iconic character of the Realms, really shouldn't be present in the Table-Top setting. So with the death of Mystra, she imparts her essence unto Elminster and he rises as a deity and his physical presence removed from the Prime Material.
As per the rules set..well as for now I am going to try to develop the world free of any rules assumptions. But if needed I will be falling back on Pathfinder and injecting it with elements from The Collected Book of Experimental Might. Since magic has been rent and reassembled, I like the idea of spell levels ranging from 1-20.
I do like the idea of the Spellscarred and may implement it in a way similar to Dragonmarks. I also love the idea of the Lightning Rail (Eberron) and may very well add it as well, probably not continent spanning...but highly localized to a small(ish) region...probably The Sword Coast.
That is all I have for now. Will add more to this inspiration and time permitting.